Differential immunolocalization of m2 and m3 muscarinic receptors in the anteroventral and anterodorsal thalamic nuclei of the rat

In this study, to identify the precise localization of m2 and m3 muscarinic receptors in the anteroventral and anterodorsal thalamic nuclei of the rat, we used receptor-subtype-specific antibodies and characterized their immunolocalization patterns by light and electron microscopy. Many m2-positive neurons were distributed throughout these nuclei. Ultrastructural analysis showed that more than 30% of m2-positive dendritic profiles in these nuclei are proximal dendritic shafts. Moreover, a few m2-positive fiber terminals were found only in the anterodorsal thalamic nucleus. These m2-positive terminals were large (1.10+/-0.30 mum in diameter) and formed asymmetrical synapses with dendritic profiles. The m3-positive neurons were also distributed in both nuclei, and the m3-positive neuropil exhibited a significant staining,gradient, with the most intense staining in the ventrolateral part of the anteroventral thalamic nucleus. This region receives the densest cholinergic input originating from the dorsal tegmental region. At the ultrastructural level, the majority of m3-positive dendritic profiles were more distal regions of the dendrites compared to the m2 receptors in the anteroventral thalamic nucleus. However, no significant difference in the intradendritic distribution pattern between m2 and m3 receptors was found in the anterodorsal thalamic nucleus, which receives no cholinergic input. These findings show the differential localization of m2 and m3 receptors in the anteroventral and anterodorsal thalamic nuclei, and suggest that the m3 receptors are spatially more closely associated with ascending cholinergic afferent fibers in the anteroventral thalamic nucleus. (C) 2001 Elsevier Science B.V. All rights reserved.